218 NEW REFLECTING QUADRANT. 



IV. 



Defcription of a new Reflecting Quadrant. By Mr. Ezekiel 

 Walker. From the Author. 



To Mr. NICHOLSON. 



SIR, 



Ofefervatlonson J. HAT inflrument which goes by the name of Hadley's 

 drant. S 9Ua " Quadrant, may perhaps be deemed one of the moft ufeful in- 

 ventions of the laft century. Although its arc is only the one 

 eighth part of a circle, yet it is fo conftructed as to meafure 

 angles from 0° to 90° by the fore obfervation, and from 90° 

 to 180° by the back obfervation. But as no method has yet 

 been found out, by which the back horizon glafs can be ad- 

 jured with the fame exactnefs as the fore one, all angles above 

 90° which are taken by the back obfervation, cannot be re- 

 lied on fo much as thofe that are taken by the fore obferva- 

 tion. To obviate this inconvenience, the fextant was invent- 

 ed, by which any angle lefs than 120° may be taken by the 

 fore obfervation. This is an invaluable inflrument to the 

 nautical aftronomer ; and indeed it is much to be regretted, 

 that its ufe is not fo generally underftood by travellers, as the 

 Imperfe&ions of. imperfect ftate of geography requires. It mull, however, be 

 the fextafit. admitted, that it is not quite free from imperfections. Firft, 

 an angle greater than 120° cannot be taken by it ; and fe- 

 condly, when a large angle is taken, the rays of light fall fo 

 obliquely upon the index glafs as may occalion fome doubt re- 

 fpecting the truth of the obfervation. Mr. Ludlam gives it as 

 a general rule, in conftructing an octant, " that very oblique 

 reflections from the mirrors ought to be avoided*." If it be 

 neceflary to obferve this rule in conftructing an octant, the fame 

 mould be attended to, as much as poffible, in taking obfer* 

 vations. 

 New inftrument Thefe imperfections fuggefted the idea of an inftrument to 



meafuring to meafure any angle between 0° and 180° by the fore obferva- 

 i8o* without . • , , t. „ _. r , • ™ 



oblique reflec- tl0n > without very oblique reflections from the mirrors. Plate 



lion* XII. Fig. 2. exhibits a plan of this inflrument, where LM C 



reprefents an octant, conftructed in the ufuai way, A B the 



* Ludlam on the ufe of Hadley's quadrant, par. 91. 



index 



